Internal combustion engines, e.g., spark ignition engines, diesel engines, and hybrid internal combustion engines, have a cylinder block and a cylinder head which are connected to one another to form the individual cylinders (combustion chambers). In some examples, such an engine may include at least four cylinders.
The cylinder head conventionally serves to hold the valve train. To control charge exchange, an internal combustion engine may include various control elements—e.g., lifting valves—and actuating devices for actuating the control elements. The valve actuating mechanism employed for the movement of the valves, including the valves themselves, is referred to as the valve train. During the charge exchange, combustion gases may be discharged via outlet openings of the cylinders, and the charging of the combustion chambers, e.g., the induction of fresh mixture or fresh air, may take place via inlet openings in the cylinders.
In some examples, the exhaust lines of the cylinders may merge to form one common overall exhaust line. In other examples, the exhaust lines of the cylinders may be grouped to form two or more overall exhaust lines. The merging of exhaust lines to form an overall exhaust line is referred to in general and within the context of the present invention as an exhaust manifold, and the part of the overall exhaust line which lies upstream of a turbine arranged in the overall exhaust line belongs to the exhaust manifold.
The selected configuration of the merging cylinder exhaust lines may be dependent on which operating range has priority in the design of the internal combustion engine. For example, the selected configuration of the exhaust lines may depend on which operating ranges the operating behavior of the internal combustion engine should be optimized for.
The inventors herein have recognized that so-called impulse supercharging may be desired in supercharged internal combustion engines which are equipped with at least one turbine on the exhaust-gas side where a satisfactory operating behavior in the low rotational speed or load range, e.g., at relatively low exhaust-gas quantities, is desired.
Here, the dynamic wave phenomena which may occur in the exhaust-gas discharge system—in particular during the charge exchange—may be utilized for the purpose of supercharging and for improving the operating behavior of the internal combustion engine.
The evacuation of the combustion gases out of a cylinder of the internal combustion engine during the charge exchange is based substantially on two different mechanisms. If the outlet valve opens when the cylinder piston is near bottom dead center at the start of the charge exchange, the combustion gases flow at high speed through the outlet opening into the exhaust system due to the high pressure level present in the cylinder at the end of the combustion and the associated high pressure difference between the combustion chamber and the exhaust line. This pressure-driven flow process is assisted by a high pressure peak which is also referred to as a pre-outlet shock and which propagates along the exhaust line at the speed of sound, with the pressure being dissipated or reduces to a greater or lesser extent with increasing distance traveled as a result of friction.
During the further course of the charge exchange, the pressures in the cylinder and in the exhaust line are equalized, such that the combustion gases are no longer evacuated primarily in a pressure-driven manner but rather are discharged as a result of the reciprocating movement of the piston.
At low loads or rotational speeds, e.g., at low exhaust-gas quantities, the pre-outlet shock may advantageously be utilized for impulse supercharging, as a result of which it is possible to obtain high turbine pressure ratios even at low turbine rotational speeds. By means of exhaust-gas turbocharging, it is possible in this way to generate high charge pressure ratios, e.g., high charge pressures, even at low exhaust-gas quantities, e.g., at low loads and rotational speeds.
Impulse supercharging may be particularly advantageous for accelerating the turbine rotor, e.g., increasing the turbine rotational speed, which may be substantially reduced during idle operation of the internal combustion engine or at low load, and which should frequently be increased again with as little delay as possible by means of the exhaust-gas flow in the event of an increased load demand. The inertia of the rotor and the friction in the shaft bearing arrangement generally slow an acceleration of the rotor to higher rotational speeds and therefore hinder an immediate rise in the charge pressure.
The inventors herein have recognized that, in order to be able to utilize the dynamic wave phenomena occurring in the exhaust-gas discharge system, in particular the pre-outlet shocks, for supercharging and for improving the operating behavior of the internal combustion engine, the pressure peaks or pre-outlet shocks in the exhaust system must be maintained. Thus, it may be expedient for the exhaust lines or cylinders to be grouped in such a way that the pre-outlet shocks of the individual cylinders in the exhaust-gas discharge system are maintained.
A cylinder head in which the cylinders are grouped is therefore also a subject of the present invention. According to the invention, at least four cylinders are configured in such a way as to form two groups, where each group includes at least two cylinders. The exhaust lines of the cylinders of each cylinder group merge in each case to form an overall exhaust line so as to form an exhaust manifold, specifically in such a way that the dynamic wave phenomena in the exhaust lines of a cylinder group have the least possible adverse effect on one another.
The two overall exhaust lines can then be supplied separately from one another in each case to the turbine of an exhaust-gas aftertreatment system, or else to a twin-flow turbine.
According to the invention disclosed herein, a twin-flow turbine, which includes an inlet region with two inlet ducts, may be used for supercharging the internal combustion engine. The two overall exhaust lines are connected, separately from one another, in each case to an inlet duct of the twin-flow turbine. The two exhaust-gas flows conducted in the overall exhaust lines are merged downstream of the turbine or while flowing through the rotor of the turbine, but not upstream of the turbine.
If the cylinders or exhaust lines are grouped such that the pre-outlet shocks are maintained for impulse supercharging, a twin-flow turbine in particular is suitable for supercharging.
The use of a twin-flow turbine instead of two separate turbines offers advantages with regard to the densest possible packaging in the engine bay and with regard to the costs of the drive unit. In some examples it may be desirable to arrange the turbine as close to the engine as possible in order to ensure the highest possible exhaust-gas enthalpy at the inlet into the turbine, in order to improve the response behavior of the exhaust-gas turbocharger and to keep the path of the hot exhaust gases to the different exhaust-gas aftertreatment systems as short as possible. A twin-flow turbine has may provide such advantages on account of the restricted spatial conditions.
However, the inventors herein have recognized that said supercharging of the internal combustion engine by means of a twin-flow turbine, which is known from the prior art, has room for improvement. As already mentioned above, the exhaust-gas pressure, in particular the pre-load shock, may be dissipated to a greater or lesser extent along the exhaust line with increasing distance traveled as a result of friction. In some examples, the grouped merging of the exhaust lines of the cylinders may result in two exhaust manifolds with different-sized exhaust-gas volumes. For example, the exhaust lines of the two manifolds may be of different lengths and differ in terms of line guidance, generally being curved to different degrees and with different frequencies.
Such differences may lead to different pressure profiles p(t) in the exhaust gas at the two manifold outlets, and, in particular, to different-sized pressure peaks at the manifold outlets, i.e., at the inlet into the two corresponding inlet ducts of the twin-flow turbine, and consequently to different-sized pressure peaks at the outlet of the two inlet ducts of the twin-flow turbine, i.e., at the inlet into the rotor.
The inventors herein have recognized that the different-sized pressure peaks at the inlet into the rotor may lead to a reduction in turbine efficiency. Thus, in order to be able to operate the turbine provided in the exhaust system optimally, e.g., as efficiently as possible, the pressure peaks at the inlet into the turbine, i.e., at the inlet into the rotor of the turbine, should be substantially equal in size.
In general, in a cylinder head having four cylinders in an in-line arrangement the exhaust lines of the two outer cylinders as a first cylinder group merge to form a first overall exhaust line and the exhaust lines of the two inner cylinders as a second cylinder group merge to form a second overall exhaust line.
Said configuration of the cylinders makes allowance for the fact that the cylinders of a four-cylinder in-line engine are generally ignited in the sequence 1-3-4-2, with the cylinders being numbered successively in series starting from an outer cylinder of the row of cylinders. This proposed grouping of the cylinders ensures that the two cylinders both of the first cylinder group and also of the second cylinder group have an ignition interval of 360° CA, where CA denotes piston crank angle. The two cylinders of each cylinder group therefore have the greatest possible offset with regard to their working processes, which is advantageous with regard to maintaining the pre-outlet shocks.
In some examples, the exhaust manifold of the two outer cylinders in the configuration described above, may have a larger exhaust-gas volume than the exhaust manifold of the two inner cylinders.
The inventors herein have recognized that, as a result, the pressure peak which results from a pre-outlet shock in the overall exhaust line of the second manifold, i.e., at the outlet of the second manifold, may be higher than the pressure peak in the overall exhaust line of the first manifold, i.e., at the outlet of the first manifold. However, according to the prior art, the inlet ducts of the turbine are of equal size.
The inventors herein have recognized that in order to improve the overall efficiency of a twin-flow turbine, the range of fluctuation of the section pressure ratio should be minimized, which may be achieved by means of aligning the pressure peaks associated with each manifold.
Against the background of that stated above, it is an object of the present invention to provide an internal combustion engine as per the preamble of claim 1, e.g., of the generic type, which is optimized with regard to the operation of the twin-flow turbine.
It is a further object of the present invention to specify a method for operating an internal combustion engine of said type.
The first object may be achieved by means of a supercharged internal combustion engine which has at least one cylinder head with at least four cylinders, each of which has at least one outlet opening which is adjoined by an exhaust line for discharging the exhaust gases out of the cylinder, with at least four cylinders being configured in such a way as to form two groups with at least two cylinders, and with the exhaust lines of the cylinders of each cylinder group merging in each case to form an overall exhaust line such that an exhaust manifold is formed, in such a way that the two exhaust manifolds have different-sized exhaust-gas volumes, and at least one twin-flow turbine which has an inlet region with two inlet ducts, with, in each case, one of the two overall exhaust lines opening out into one of the two inlet ducts, and which is characterized in that the inlet ducts of the twin-flow turbine are of different sizes, with different-sized cross sections and/or different-sized exhaust-gas volumes, and the overall exhaust line of the exhaust manifold with the smaller exhaust-gas volume is connected to the larger inlet duct, and the overall exhaust line of the exhaust manifold with the larger exhaust-gas volume is connected to the smaller inlet duct.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.